This invention relates to a filter medium applied for cleaning air in e.g., a clean room, and an air filter unit using the same. More specifically, this invention relates to a filter medium applied for cleaning air in manufacturing electronic members such as semiconductors and liquid crystals, and an air filter unit using the same.
As filters applied for cleaning air in e.g., a clean room, the inventors of this invention have already disclosed a polytetrafluoroethylene (referred to as PTFE hereinafter) porous film (for example, Japanese Patent Application Tokkai Hei 5-202217). In addition, the lamination of a thermoplastic material such as a span bond nonwoven cloth made of sheath-core structured long fibers onto both the surfaces of the PTFE porous film was also proposed so as to protect the porous film from scratches and pin holes (Japanese Patent Application Tokkai Hei 6-218899).
However, the inventors of this invention found in accordance with the Standard of Test Method for Combustion of Air Filter Media (JACA No. 11-1977) that the prior art mentioned above has unsatisfactory flame-resistant properties.
A flame-resistant filter medium, and an air filter unit using the same are sometimes fixed to a ceiling as equipment. In this case, particularly, flame-resistant properties are required so as to prevent fire.
This invention aims to solve the above-mentioned conventional problems by providing a flame-resistant filter medium and an air filter unit using the same.
In order to achieve the above purposes, the flame-resistant filter medium of this invention is a filter medium comprising a gas permeable supporting material on at least one surface of a PTFE porous film and which has 150 mm or less maximum carbonized length based on the Standard of Test Method for Combustion of Air Filter Media (JACA No. 11-1977). The flame-resistant properties in this invention are measured by the Standard of Test Method for Combustion of Air Filter Media (JACA No. 11-1977) prepared by the Filter Standard Committee of JAPAN AIR CLEANING ASSOCIATION (JACA).
It is preferable that the air permeable supporting material of the flame-resistant filter medium is a flame-resistant air permeable supporting material.
The PTFE porous film can be one or more layers in the flame-resistant filter medium of this invention. As the flame-resistant filter medium, an air permeable supporting material is applied on one surface of the PTFE porous film, on both surfaces of the porous film, or is alternately or randomly applied altogether with the porous film. Particularly, the filter medium in which the supporting material is applied on both surfaces of the porous film is preferable.
There is no particular limitation on the PTFE porous film, and a conventional PTFE porous film may be applied to the flame-resistant filter medium. A PTFE porous film that can provide the efficient scavenging of floating fine particles and has properties such as pressure loss (the same or better properties than an HEPA filter and ULPA filter) required for a filter unit applied in a clean room for manufacturing semiconductors, liquid crystals, etc. and in manufacturing devices is especially preferable. For instance, pressure loss is preferably 10-100 mmH2O when air is permeated at the flow velocity of 5.3 cm/second, and the scavenging efficiency of 0.10-0.12 xcexcm dioctylphthalate (DOP) is preferably 99.0% or more. The PTFE porous films with such properties are described in detail in Japanese Patent Application Tokkai Hei 5-202217 and WO 94/16802, the disclosure of which are incorporated herein by reference.
The above-noted PTFE porous films applied in this invention can easily be prepared by a conventional method. The method includes, for example, the steps of extruding a paste of a PTFE fine powder and an extrusion assistant, providing a tape by pressing and spreading the extruded paste, and stretching out the non-baked or semi-baked paste in two axial directions. This method is specifically explained in Tokkai Hei 5-202217, WO 94/16802, etc.
In the flame-resistant filter medium of the invention, the air permeable supporting material is preferably a flame-resistant air permeable supporting material, or more preferably an organic-particularly, resinous-flame-resistant air permeable supporting material.
The air permeable supporting material of the flame-resistant filter medium is preferably a nonwoven fabric or woven cloth, or more preferably a resinous nonwoven fabric. Air permeable supporting materials made of glass fibers such as glass fiber nonwoven fabric, glass fiber paper and a glass fiber air filter medium are not preferable since they generate boron (B).
It is also preferable that the air permeable supporting material of the flame-resistant filter medium is made of at least one material selected from the group consisting of polyester and polyamide. It is further preferable that the air permeable supporting material is made of polyester fibers, and contains no polyolefin. The polyester mentioned above indicates polyethylene terephthalate (PEI), polybutylene terephthalate (PBI), and the like.
A flame retardant in the flame-resistant filter medium is preferably copolymerized. It is preferable that the flame retardant is copolymerized particularly to polyester fibers. In adding flame-resistant properties by e.g., organic phosphorous compounds, phosphorus (P) would not be detected from a filter medium if the flame retardant is copolymerized.
The polyester fiber material of the flame-resistant filter medium may be filaments nonwoven fabric.
The air filter unit of this invention uses a filter medium having an air permeable supporting material at least on one surface of the PTFE porous film. The filter medium is bent in wave forms and kept in the supporting body (frame), and the periphery is sealed. The filter medium has 150 mm or less maximum carbonized length according to the Standard of Test Method for Combustion of Air Filter Method (JACA No. 11-1977).
Conventional methods of kneading, copolymerizing or coating compounds having flame-resistant properties onto the fiber surface may be used as a means to add flame-resistant properties. For example, polyester fibers become flame-resistant after a flame retardant such as an organic phosphorous compound is copolymerized or kneaded to the fiber, or the fiber can be given flame-resistant properties by polymer blend or the like. More specifically, the following methods can be applied.
(1) Use of Flame-Resistant Materials
Flame-resistant fibers having flame-resistant properties in themselves include organic or inorganic fibers such as aromatic polyamide fibers, modified acrylic fibers, flame-resistant polynosic, flame-resistant vinylon, flame-resistant polyester, oxidation acrylic fibers (flame proof fibers), rayon carbonized fibers (flame proof fibers), aramid fibers, polyarylate fibers, phenol fibers, polybutylene isocyanate fibers (PBI fibers), polyvinylidene chloride fibers, asbestos, carbon fibers, metalfibers, and silica fibers, PTFE fibers, tetrafluoroethyleneperfluoroalkylvinylether copolymer (PFA) fibers, and polyvinylidene fluoride (PVdF) fibers.
(2) Flame-resistance Treatments
Flame-proof and flame-resistant properties are added by treatments such as a method of applying a flame retardant by a dipping or a coating process after preparing a thread, woven cloth, or nonwoven fabric from fibers.
In case of synthetic fibers, the treatments include a method of introducing a flame retardant by chemical bonding such as covalent bonding in preparing polymer, and a method where the fibers are kneaded in polymer or blended with polymer in melting, extruding and spinning processes.
The flame retardant includes inorganic flame retardants such as aluminum hydroxide, decabro-based bromine flame retardants such as decabromodiphenyloxide, nondeca-based bromine tetrabromobisphenol, chlorine-based flame retardants, and phosphorous flame retardants.
By mixing (spinning) flame-resistant and non-flame resistant fibers together, a thread, woven cloth or nonwoven fabric as a whole can be provided with stronger flame-resistant properties. The examples are a material in which flame-resistant acrylic fibers and cotton are spun together (DELAILA manufactured by Kurabo Industries Ltd.), and a nonwoven fabric (TEISEN PAIKU manufactured by Teikoku Seni Co., Ltd.) in which carbonized fibers (pre-carbon) and aramid fibers (Kevler) are mixed and spun.
The following have added flame-resistant properties: a nonwoven fabric in which polyvinylidene (PVC) and polyethylene (PE) are mixed, and a thread in which polyvinylidene fluoride (PVdF) and polyester fibers are mixed and spun.
When polyester fibers are used for the filter medium as a material, the material is preferably a nonwoven fabric, particularly a nonwoven fabric made of filaments (filament fibers). The examples are a PET nonwoven fabric in which a flame retardant such as phosphorus (P) compounds is copolymerized, a nonwoven fabric of sheath-core fibers comprising a high melting point PET (core section) and a low melting point PET (sheath section) where the above-noted flame retardant is copolymerized, a nonwoven fabric of sheath-core fibers comprising a high melting point PET (core section) and PBT (sheath section) where the above-noted flame retardant is copolymerized, and the like. When nonwoven fabrics are made from filaments, the fabrics are formed in melting and spinning processes (spinning direct nonwoven fabric), so that the fabrics are kept in a clean condition from the beginning. Nonwoven fabrics can be prepared from filaments by e.g., the span bond method, flash spinning method, melt-blowing method, or the like.
A card has to be passed through for opening when short fibers are used for a nonwoven fabric. In order to facilitate the passage, oil is added. Therefore, the oil has to be removed from the fabric in preparation for usage. The unwoven fabrics are formed from short fibers by the needle punch method, water jet method, stitch bond method, or the like.
It was found that a filter medium having a flame-resistant air permeable supporting material (nonwoven fabric made of polyester fibers, particularly, nonwoven fabric made of filaments) on at least one surface of a PTFE porous film generated little total organic carbon (mentioned as TOC hereinafter). In this invention, TOC indicates the total of various gaseous organic materials such as dodecane, tridecane, dioctylphthalate, siloxane and the like. Little generation of TOC and inorganic materials (such as phosphorus in a cleaned air space such as a clean room) can improve the qualities of semiconductors, liquid crystals, and the like in manufacturing processes.
In the filter medium, a PTFE porous film and a flame-resistant air permeable supporting material can be adhered to each other by a hot-melt adhesive, so that the porous film and the supporting material would be in one body. In addition, with the hot-melt adhesive, the generation of TOC is reduced.
Particularly, a PTFE porous film and an air permeable supporting material (nonwoven fabric made of polyester fibers in which a flame retardant is copolymerized, especially filaments nonwoven fabric) bonded by a polyester-based hot-melt adhesive are preferable.
Polyester fiber materials adhered by the hot-melt adhesive include high melting point PET fibers, low melting point PET fibers, nonwoven fabrics made of mixed fibers of high melting point PET fibers and low melting point PET fibers, nonwoven fabrics of sheath-core fibers comprising a high melting point PET (core section) and a low melting point PET (sheath section), nonwoven fabrics of sheath-core fibers comprising a high melting point PET (core section) and a PBT (sheath section), and the like.
Adhering methods include conventional methods, or more preferably a spray coating method, a spiral spray coating method, a slot spray coating method, a melt blown coating method, a print wheel coating method, a ribbon rip coating method, and the like so as to maintain the permeability of the air permeable supporting material.
The nonwoven fabrics are made of polyester fibers in which a flame retardant (for example, phosphorous compounds) is copolymerized. Nonwoven fabrics comprising at least PBT such as PBT fiber nonwoven fabrics, nonwoven fabrics of sheath-core fibers comprising PET (core) and PBT (sheath), and mixed nonwoven fabrics of PET fibers and PBT fibers can be adhered to a PTFE porous film by heat (heat roller) at temperature below the melting point of an air permeable supporting material. In this case, no adhesive is used. It is particularly preferable since there is little generation of TOC.
It is preferable that mini-pleated filter filaments having a band or ribbon spacer made of a hot-melt adhesive are kept in the above-mentioned filter unit. Since the air filter unit has a mini-pleated structure, the entire area of the filter can be used efficiently used, so that the filter preferably can be applied to the manufacture of electronic devices or machine such as semiconductors, liquid crystals and the like.
In the above-noted air filter unit, a sealing section between the supporting body and the filter medium is sealed with a hot-melt adhesive. Little TOC is generated from a hot-melt adhesive.
As described above, this invention provides a filter medium with excellent flame-resistant properties and an air filter unit using the same.
Mainly, a PTFE porous film is used for an air cleaning filter medium and an air filter unit using the same.